A sodium-led chloride spring with bicarbonate softness layered in, classified as a high-temperature water. Weakly alkaline and gentle on the skin, it carries a faint rotten-egg note of H₂S at the source, fine clinging bubbles and a barely-there salty tang. A generous 148 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother. It is served as untreated source flow with no dilution or heating, and the posted indications cite neuralgia, rheumatism and muscle ache.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺397mgP70 national89%
- Magnesiummild; a laxative if you drink itMg²⁺25.0mgP73 national6%
- Potassiumminor; matters for drinking limitsK⁺31.3mgP76 national4%
- AmmoniumNH₄⁺0.90mgP64 national0.4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺0.10mgP0 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- StrontiumSr²⁺0.10mgP26 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- BariumBa²⁺0.0000mg
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- LithiumLi⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻459mgP73 national67%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻261mgP61 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻93.5mgP53 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.6mgP40 national1%
- BromideBr⁻1.5mgP67 national0.1%
- FluorideF⁻0.40mgP31 national0.1%
- BO2BO₂⁻1.4mgP68 national
- AsO2AsO₂⁻0.0000mg
- Hydrogen phosphateHPO₄²⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- HydroxideOH⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃148mgP67 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.7mgP48 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂27.3mgP59 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.